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Web skærmformat. - Aarhus Universitet

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III. Potensrækker – 1. l’Hospitals regel og uegentlige integraler 253<br />

1.8. l’Hospitals regel 2 ☞ [S] 4.5 Indeterminate forms and l’Hospital’s rule<br />

Sætning (l’Hospitals regel)<br />

Antag, at f,g er differentiable og g ′ (x) = 0 for x = a tilpas nœr a. Hvis<br />

f(x)<br />

lim<br />

x→a g(x)<br />

er et ubestemt udtryk af form ∞<br />

∞ , så er<br />

f(x) f<br />

lim = lim<br />

x→a g(x) x→a<br />

′ (x)<br />

g ′ (x)<br />

1.9. Prøv reglen ☞ [S] 4.5 Indeterminate forms and l’Hospital’s rule<br />

Eksempel (6)<br />

er ubestemt af form ∞<br />

∞ .<br />

Heraf fås<br />

lim<br />

x→0<br />

ln |x|<br />

lim<br />

x→0 x−1 f(x) = ln |x|,f ′ (x) = |x| −1<br />

g(x) = x −1 ,g ′ (x) = −x −2<br />

ln |x|<br />

= lim<br />

x−1 x→0<br />

|x| −1 x<br />

= lim −x = 0<br />

−x−2 x→0 |x|

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